The present invention relates to video sharpness circuits and more particularly to an adjustable sharpness circuit.
It is known to improve the visual appearance of an image by increasing its apparent sharpness. This is done by decreasing the rise time of amplitude transitions in the video signal representing the image. The amplitude transitions correspond to edges in the video image so that shorter rise times correspond to sharper edges.
The basic sharpness circuit is disclosed in U.S. Pat. No. 2,851,522 issued Sept. 9, 1958 to John M. Hollywood, and in the article by Goldmark and Hollywood, A New Technique For Improving The Sharpness Of Television Pictures, Proceeding of the I.R.E., October, 1951, pgs. 1314 to 1322. This circuit is a nonlinear circuit which adds the video signal and a compensating signal obtained from the derivative of the video signal. The compensating signal has a short rise time so that the sum signal has a shorter rise time than the input video signal. In both of these documents the problem of noise immunity is not addressed.
As digital techniques are applied more frequently in television receivers it will become increasingly necessary to design receiver circuits to take into account signal quantization noise. After a digital video signal has been converted back to an analog video signal, the resulting analog video signal will have a noise component arising from its previous quantization. Circuits, such as sharpness circuits, which will process the analog video signal containing quantization noise will have to be immune to the quantization noise.
Accordingly, it is an object of the invention to provide a video sharpness circuit which performs well with analog video signals containing quantization noise.
It is another object of the invention to provide an adjustable video sharpness circuit which develops an output signal that is dependent on the input signal amplitude in an adjustable non-linear manner.